This invention relates to high pressure gas discharge lamps and more particularly to an improved mounting structure for high pressure discharge lamps employing ceramic arc tubes. Many high pressure sodium-mercury discharge lamps of the ceramic arc tube type of present manufacturer employ a "floating bridge" design at the top end of the lamp. This structure is somewhat similar to that disclosed in FIG. 4 of U.S. Pat. No. 3,623,134 with the exception that the cross-bar or "bridge" is welded to the tab on the end cap and permitted to slide with respect to the frame at the interconnection of the bridge and frame. The current structure then, consists of two parallel frame wire supports and a bridge of stainless steel slidable on the frame wires at its extremities and welded to the niobium tab on a nontubulated end cap, or to the tubulation should it be at that end of the lamp, adjacent its center. There is also a braided nickel ribbon welded both to the tab and to the frame wire to provide positive electrical connection between the upper end of the arc tube and the arc tube frame wires. The mass of the stainless steel bridge is substantial as well as that of the braided nickel ribbon which has a thermal conductivity of as high as 36 Btu/hr/sq.ft/.degree. F/ft. with the tab conductivity being as high as 42. The heat from the end cap at the upper end of the lamp is readily dissipated, when the lamp is turned off, by conduction from the cap to the niobium tab to the bridge and braided ribbon and from there to the large wire frame. Radiation of heat from all of these members cools the upper end cap quickly and causes the sodium-mercury vapor to collect there as an amalgam. Since the lower end cap with the tubulation only involves a heat sink through a niobium connector wire to the lead in conductor of the stem, the bottom cap does not cool as rapidly. This lower support structure at the base end of the lamp is substantially as that disclosed in U.S. Pat. No. 3,855,494.
With the heat being dissipated from the upper end cap due to the much larger heat sink, most of the mercury and sodium in the vapor state condenses out at the top cap when the lamp is extinguished. Upon relighting the lamp, the arc has a tendency to strike the sodium amalgam as it melts and runs down from the top cap along the alumina side wall. This action subjects the polycrystalline alumina or sapphire arc tube to severe thermal stress which in turn can cause cracks in the polycrystalline alumina if the arc tube body is of marginal quality.
Accordingly, it is the object of this invention to collect most of the sodium-mercury vapor, as it condenses, at the bottom end of the arc tube when the lamp is turned off and is cooling down. Of course, it will be apparent, that this invention as disclosed is specifically directed to lamps which are designed to function in a base down orientation. Alternatively it will be equally apparent that the concepts of this invention could be practiced in a lamp designed for base up operation by merely reversing the structure to provide for a larger heat sink at the end of the lamp remote from the base.